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Why would you get excited about a new measurement of the Boltzmann constant that took six years to achieve?

In the case of the UK scientists who made the measurement at the UK's National Physical Laboratory (NPL), it's because the long-held standard for temperature measurement is problematic at extremes.

The kelvin and the degree Celsius are currently both defined using what's known as the triple point of water – “the point at which liquid water, solid ice and water vapour can all exist in equilibrium”, the NPL says in a statement.

The triple point is a definition rather than a measurement: the three states of water can be calculated to exist at 0.01°C and a partial vapour pressure of 611.73 pascals – in kelvin, 273.16K. All other measurements of temperature are made relative to this value.

However, as Dr Michael de Podesta of the NPL explains, “The further away one measures from the temperature of the triple point of water, the harder it gets to precisely determine the ratio of exactly how much hotter or colder the temperature is than the standard temperature. This adds uncertainty to temperature measurements on top of the normal practical difficulties.”

The paper he and his collaborators (from Cranfield University and the Scottish Universities Environmental Research Centre, SUERC) have prepared for the journal Metrologia focuses on the Boltzmann constant: the relationship between a particle's temperature and its energy. The paper sets the constant as 1.38065156 (98) x 10-23 J K-1 with (98) expressing the uncertainty of the last two digits in the value.

It's those last two digits that matter: the uncertainty, which amounts to 0.7 parts per million, is around half of that in previous measurements.

The measurement was made using acoustic thermometry: an acoustic resonator was chilled to the triple point of water, and filled with a high purity argon gas. High-accuracy measurements of the speed of sound in the gas provides a measurement of the average speed of the argon molecules, from which their kinetic energy – and therefore the Boltzmann constant – could be calculated.

The resonating chamber also needed to be measured to high accuracy, using microwaves that provided an overall uncertainty of just 600 atoms' worth, or 11.7 nanometres.

Since the same constant is useful in calculating the thermal voltage of semiconductors, The Register supposes the measurement might prove useful beyond the world of basic science. ®